# chmem(1)

## NAME

         chmem - change memory allocation

## SYNOPSIS

         chmem [+] [-] [=] amount file

## EXAMPLES

         chmem =50000 a.out  # Give a.out 50K of stack space

         chmem -4000 a.out   # Reduce the stack space by 4000 bytes

         chmem +1000 file1   # Increase each stack by 1000 bytes

## DESCRIPTION

         When a program is loaded into memory, it is allocated enough  memory  for
         the text and data+bss segments, plus an area for the stack.  Data segment
         growth using malloc , brk , or sbrk eats up stack space from the low end.
         The  amount  of  stack  space  to allocate is derived from a field in the
         executable program's file header.  If the combined stack and data segment
         growth exceeds the stack space allocated, the program will be terminated.

         It is therefore important to set the amount of stack space carefully.  If
         too  little is provided, the program may crash.  If too much is provided,
         memory will be wasted, and fewer programs will be able to fit  in  memory
         and  run  simultaneously.   MINIX  does  not swap, so that when memory is
         full, subsequent attempts to fork will fail.  The compiler sets the stack
         space  to  the  largest  possible value (for the Intel CPUs, 64K - text -
         data).  For many programs, this value is  far  too  large.   Nonrecursive
         programs  that  do  not call brk , sbrk , or malloc , and do not have any
         local arrays usually do not need more than 8K of stack space.

         The chmem command changes the value of the header field  that  determines
         the  stack  allocation,  and thus indirectly the total memory required to
         run the program.  The = option sets the stack size to a  specific  value;
         the  +  and  -  options  increment and decrement the current value by the
         indicated amount.  The old and new stack sizes are printed.

## SEE ALSO

         install(1), brk(2).